It is known that certain cations and anions have corrosion inhibiting properties and that compounds containing them can be included in anti-corrosive compositions that are intended to provide adhesion and corrosion inhibiting properties to metallic surfaces and structures. Typical examples include cations of calcium, magnesium, strontium, barium, aluminium, manganese, zinc, lead, chromium, cerium and other rare earth elements together with anions such as silicate, metaborate, borate, borosilicates, chromate, molybdate, nitrophthalate, phosphate, hydrogen phosphate, phosphite, polyphosphate, phosphosilicates, phosphonates and phosphonocarboxylates.
The cations may be combined with the anions to form slightly soluble metal salts. Cations or anions may also be chemically combined with inorganic oxides such as those based on silica or alumina to produce ion-modified anti-corrosive compounds. Additionally, cations of some metals such as lead and zinc in the form of their oxides are able to enhance the anti-corrosive properties of anti-corrosive compositions. Typical examples are lead monoxide, red lead and zinc oxide.
Of course, the classic inhibitors based on lead compounds and chromates are well-known to be associated with environmental and health and safety concerns and have largely been phased out, although there are still applications where chromates continue to be used, such as in anti-corrosive coatings for aircraft applications and particular applications within coil coatings to do with exterior buildings or metallic substrates such as Galvalume, providing continued motivation for further development in the field of anti-corrosion.
Many of the replacements for lead and chromium based compounds have however in practice been based on other heavy metals such as zinc, raising further concerns about health and safety and the environment. Rare earth elements such as cerium are thought to provide pigments that are less objectionable than those based on chromium, whereas unlike calcium and magnesium, concerns may still exist if strontium and barium compounds are considered, even though as Group IIA elements, these are not regarded as heavy metals.
As mentioned above, the inhibitive compounds may be in the form of sparingly water-soluble salts and can for example be prepared by a process of particle growth and precipitation from slurries or solutions in the presence of the required cations and anions under suitable conditions. Typical examples of lead and chromate-free inhibitive compounds based on sparingly soluble salts and processes for preparing them can be found in or referenced by U.S. Pat. Nos. 4,247,526, 4,139,599, 4,294,621, 4,294,808, 4,337,092, 5,024,825, 5,108,728, 5,126,074, 5,665,149, 6,083,308 and 7,220,297 together with U.S. Patent Application Nos. 2007/0012220, 2007/0012220 and 2004/0168614 as well as GB Patents Nos. 825,976, 914,707, 915,512, 1,089,245, DE Patents Nos. 2,849,712, 2,840,820, and 1,567,609 and EP Patent No. 522,678, the entire subject matter of which is incorporated herein by reference.
The inhibitive compounds may also be in the form of particles of inorganic oxides such as silica, silicates, alumina and aluminosilicates comprising additional inhibitive cations and anions. These inhibitive compounds can for example be prepared by a process of precipitation or gelation of the oxide in the presence of the required cations and anions under suitable conditions. Typical examples of such inhibitive compounds and processes for making them may be found in U.S. Pat. No. 4,849,297 and GB 918,802, the entire subject matter of which is incorporated herein by reference. GB 918,802 refers to a precipitated calcium silicate having a SiO2:CaO ratio in excess of 1:1 and usually between 2:1 and 5:1. U.S. Pat. No. 4,849,297 refers to the precipitation of an amorphous calcium containing silica having a low surface area and oil absorption and a calcium content of 6 to 9% by weight expressed as CaO.
Inhibitive compounds based on inorganic oxides can alternatively be made through a process of ion-exchange, in which surface protons and hydroxyl groups of the pre-formed oxide are replaced by contacting the oxide with a solution containing the required inhibitive cations and anions, again under suitable conditions. Processes for making such exchanged oxides may be found in or referenced by U.S. Pat. Nos. 5,405,493, 4,687,595, 4,643,769, 4,419,137, 4,474,607, and 5,041,241, together with EP 0412686, the entire subject matter of which is incorporated herein by reference. Preferentially, the oxides referred to, such as silica gels are microporous, having average pore sizes of around 2 nm. U.S. Pat. No. 5,041,241 refers to a two component blend of calcium containing microporous silicas. Other examples concerning ion-exchanged aluminosilicate compounds are U.S. Pat. No. 6,139,616 and US 2004/0091963 the entire subject matter of which is incorporated herein by reference.
Of course, from the above description, combinations of inhibitive compounds based on sparingly soluble salts and those based on inorganic oxides could be prepared simultaneously in various ways according to the composition of the solution or slurry from which the inhibitive compounds are to be prepared and the processing route, allowing in principle for a great variety in properties displayed by the resulting inhibitive compound.
In many cases, the films and coatings employed in anti-corrosion have a certain permeability to water and it is believed that the mechanism of corrosion inhibition provided by the aforementioned anti-corrosive compounds involves gradual dissolution of the compounds in water, releasing ions as the active inhibitors. For such systems to be effective over a long period, the solubility of the compound is particularly important. If the compound is too soluble, blistering of the coating may occur and the compound will be rapidly depleted; if it is insufficiently soluble the compound will be ineffective. Whether the inhibitive compound is a sparingly soluble salt, or based on an inorganic oxide or is some combination of the two, the typical solubility of such compounds suitable for use in films and coatings results in inhibitive ion concentrations in aqueous media of around 10−5M to 10−2M.
For inhibitive compounds based on inorganic oxides, the inorganic oxide may itself have a certain solubility with respect to the provision of inhibitive substances, according to the nature of the environment in which the corrosion inhibiting particles are used e.g. in the case of silica, silicic acid has a background solubility of about 10−3M with the concentration of silicate being pH dependent and having a value of 10−2M for example at a pH of about 10.5.
It is however sometimes believed that these types of corrosion inhibiting particles can act to release inhibitive cations and anions into solution by ion exchange with aggressive ions existing in that environment as an additional or alternative mechanism of action to one based on dissolution. The rate of release of the corrosion inhibiting ions would then be influenced by the permeability of the film or coating to the exchanging ions in addition to or rather than dissolution of inhibitive ions into the permeating aqueous environment. Corrosion inhibiting ions would in that case be released to a greater extent from the inorganic oxide in those areas where the desired barrier properties of the coating were weakest, leading thereby to improved performance properties.
The anti-corrosive compounds referred to above are usually made available in the form of dry powders, making use of washing, drying and milling operations as required as additional processing steps and average particle sizes of the powders are usually about 1 to 2 microns or more, although can be less than 1 micron.
In many practical anti-corrosive systems such as coating formulations, combinations of anti-corrosive pigments are employed in the development of lead and chromate-free formulations, which may or may not include other heavy metal containing pigments. Apart from optimizing performance, such combinations can in some cases also allow the heavy metal content of the formulation to be reduced. Typical examples of such combinations can be found in U.S. Pat. Nos. 6,485,549, 6,890,648, 7,033,678, 7,244,780 and US Patent Application Nos: 2002/0031679, 2004/0224170, 2005/0148832, 2007/0048550 and 2007/0088111 as well as EP1172420, EP1291,453 and EP1475226 and WO2000022054, the entire subject matter of which is incorporated herein by reference.
The performance and properties obtainable from the less objectionable pigments are not however in general always at the level associated with the traditional lead and chromate containing systems and for other heavy metal containing pigments, attempting to lower or eliminate their use may introduce further compromise in performance, a situation that continues to provide motivation to find improved alternatives to the traditionally used pigments. Of increasing concern as well are the costs associated with pigment production, arising from increasing energy costs and ease of processing as well as raw material costs in relation to the cost-effectiveness of the final anti-corrosive system into which the pigments are incorporated. The present invention attempts to address these various issues.